Otto cycle engines are very inefficient. Only about one-third of the heat available in the fuel burned in the engine is delivered as work at the drive shaft. About one-third of the heat is lost in the exhaust gases. Another one-third is rejected to the coolant, lost by radiation and used to overcome mechanical friction. In by far the most common uses of Otto cycle engines, vehicles, less than optimum engine performance under most driving conditions further reduces overall efficiency in several ways.
Four-stroke Otto cycle engines perform most efficiently at or near full power (i.e., wide-open throttle) for two principal reasons. First, the throttle is wide-open, so the fuel-air charge is inducted with the minimum restriction, as compared to the pumping loss associated with a manifold vacuum condition at partial throttle settings in which a larger part of the engine power is used to induct the charge. Second, the maximum mass of fuel and air are inducted at and near wide-open throttle, so the pressure of the charge at the time of ignition and combustion is at a peak. When the mass of the charge is diminished by throttling the intake, the pressure at the time of ignition and combustion are well below the optimum for the highest thermal efficiency of the fuel combustion.
The efficiency losses discussed above have been long recognized, and many proposals have been made to reduce or eliminate them. For example, Weiss U.S. Pat. No. 3,416,502 (1968) proposes reducing the pumping loss and eliminating the throttle of an Otto cycle engine by providing variable-timed intake valves that are controlled to stay open during part of the compression stroke at less than full power so that the fuel-air mass and, therefore, power are controlled without throttling. To the same effect is Aoyama U.S. Pat. No. 4,357,917 (1982).
There are many patents for Z-crank engines in which the displacement or the clearance volume or both are varied by changing the position or angle or both of the crank arms, thereby providing a variable compression ratio in order to improve combustion efficiency. Many of the engines in this area of the patent literature inherently provide a reduced displacement volume in conjunction with an increased clearance volume, and vice versa, so whatever advantage may be gained from maintaining a controlled compression of the fuel-air charge is offset by a reduced expansion ratio. Exemplary of the U.S. patents on Z-crank engines are the following:
Eckert U.S. Pat. No. 2,465,638 (1949)
Welsh et al U.S. Pat. No. 3,319,874 (1967)
Kemper et al U.S. Pat. No. 4,144,771 (1979)
Roseby et al. U.S. Pat. No. 4,174,684 (1979)
Bex et al. U.S. Pat. No. 4,294,139 (1981)
Scalzo U.S. Pat. No. 4,433,596 (1984)
The problem of heat rejection to a coolant has been addressed recently in the widely publicized efforts to develop an "adiabatic" Diesel engine using ceramic-lined (insulating) cylinders. To date, the results are reported to be disappointing. It appears that nearly all of the reduction in heat rejection to a coolant is offset by increased heat rejection to the exhaust, with little change in the thermodynamic cycle.
It has been suggested that some of the heat lost to the coolant can be used to produce power; the following U.S. patents relate to engines that carry out six-stroke work cycles, consisting of the conventional four strokes of an Otto cycle followed by the two strokes of a steam cycle, the steam being produced by injecting water into the cylinders after the combustion-exhaust stroke:
Dyer U.S. Pat. No. 1,339,176 (1920)
Burtnett et al. U.S. Pat. No. 1,501,392 (1924)
Rohrbach U.S. Pat. No. 2,671,311 (1954)
Tibbs U.S. Pat. No. 3,964,263 (1976)
Kellogg-Smith U.S. Pat. No. 4,143,518 (1979)
There are many reasons why the concepts of the patents referred to above have not been adopted commercially. For one thing, they involve complex mechanisms and controls that add to the initial cost of producing the engine and the operating costs of maintaining it. For another, there are alternative ways of improving engine efficiency that are more readily adopted in engines of more conventional design. Examples are the higher operating temperatures afforded by higher-pressure cooling systems, higher compression ratios afforded by better cooling of the engine head (thin domes, better finish for reduced incidence of hot spots, aluminum heads), improved shaping of the combustion dome for reduced knock at high compression ratios, higher octane fuels, better control of ignition and fuel supply and many others. The various proposals discussed above, moreover, often provide but a very small improvement in efficiency that is insufficient to warrant the greater costs and the greater potential for maintenance and durability problems introduced to gain the advantage. In many cases the geometry of the engine makes it significantly less desirable than conventional designs, notably in the case of vehicle engines. Also, until recently the cost of fuel has been low enough to make it, in the final analysis, uneconomical to turn to relatively costly engine designs and support systems.